1. Field of the Invention
This invention relates to a switching power supply that inputs an AC voltage and converts it to a predetermined output, and that particularly converts power at a high power factor and with high efficiency.
2. Description of Related Art
A conventional switching power supply has a DC input power source and it is difficult to realize a high power factor (see, for example, Patent Literature 1). This example will now be described in detail with reference to FIG. 1. FIG. 1 is a structural view showing a conventional switching power supply.
In FIG. 1, a common potential COM and a common potential GND are used as a common potential of the switching power supply. An AC voltage Vac is connected to a rectifying circuit DB1. The rectifying circuit DB1 rectifies the AC voltage Vac.
The rectifying circuit DB1 is connected to a smoothing capacitor C1. The smoothing capacitor C1 smoothes an output of the rectifying circuit DB1 and generates a voltage VC1 to be a DC input power source.
A series switch circuit formed by a first switch Q1 and a second switch Q2 is provided between the two electrodes of the smoothing capacitor C1. Moreover, a series circuit formed by an inductor L41, a primary winding N41 of a transformer T41 and a resonance capacitor C11 is provided between a connection point between the first switch Q1 and the second switch Q2, and one terminal (negative electrode) of the smoothing capacitor C1.
A secondary winding N42 and a secondary winding N43 of the transformer T41 are connected to a diode D1 and a diode D2, respectively, then connected to an inductor L3 and a capacitor C3, and then connected to a load Load.
As the first switch Q1 is turned on/off and the second switch Q2 is turned on/off in a complementary manner with the first switch Q1, voltages to be an output are induced at the secondary winding N42 and the secondary winding N43 of the transformer T41. The voltages are rectified by the diodes D1 and the diode D2, then smoothed by the inductor L3 and the capacitor C3 and thus become an output voltage Vout to supply power to the load Load.
In this manner, in the conventional example of FIG. 1, the DC input power source is generated from the AC voltage Vac and the DV input power source is converted to the output voltage Vout.
As an input current Iin, a pulse-like current flows.
Moreover, a conventional switching power supply has a reverse-current blocking diode Da between a smoothing capacitor C1 and a switching regulator circuit (first switch Q1, second switch Q2, resonance capacitor C2, transformer T1, diode D11, diode D12 and capacitor C3) (see, for example, Patent Literature 2).
This example will now be described in detail with reference to FIG. 2. FIG. 2 is a structural view showing another conventional switching power supply. The same elements as those in the conventional example of FIG. 1 are denoted by the same symbols and numerals and will not be described further in detail.
In the conventional example of FIG. 2, the positive electrode of the smoothing capacitor C1 is connected to the anode of the reverse-current blocking diode Da. A series switch circuit formed by the first switch Q1 and the second switch Q2 is connected to the cathode of the reverse-current blocking diode Da. One end of the resonance capacitor C2 is connected to a connection point between the first switch Q1 and the second switch Q2.
One end of a primary winding N1a of the transformer T1 is connected to the cathode of the reverse-current blocking diode Da and the drain of the second switch Q2. That is, the one end of the primary winding N1a of the transformer T1 is not directly connected to the positive electrode of the smoothing capacitor C1.
The other end of the resonance capacitor C2 is connected to one end of a primary winding N1b of the transformer T1.
The other end of the primary winding N1a is connected to the other end of the primary winding N1b. 
A secondary winding N2 and a secondary winding N3 of the transformer T1 are connected to the diode D11 and the diode D12, respectively, and then connected to the capacitor C3 and a load Load.
As the first switch Q1 is turned on/off and the second switch Q2 is turned on/off in a complementary manner with the first switch Q1, voltages to be an output are induced at the secondary winding N2 and the secondary winding N3 of the transformer T1. The voltages are rectified by the diode D11 and the diode D12, then smoothed by the capacitor C3 and become an output voltage Vout to supply power to the load Load.
That is, the first switch Q1, the second switch Q2, the resonance capacitor C2, the transformer T1, the diode D11, the diode D12 and the capacitor C3 form the switching regulator circuit (first switch Q1, second switch Q2, resonance capacitor C2, transformer T1, diode D11, diode D12 and capacitor C3).
In this conventional example of FIG. 2, an AC voltage Vac is converted to the output voltage Vout. A harmonic component of an input current Iin is restrained.
FIG. 3 is a structural view showing still another conventional switching power supply. The same elements as those in the conventional example of FIG. 1 and the conventional example of FIG. 2 are denoted by the same symbols and numerals and will not be described further in detail.
In the conventional example of FIG. 3, one end of a resonance capacitor C11 is connected to the negative electrode of a smoothing capacitor C1, as in the conventional example of FIG. 1.
One end of a primary winding N41 of a transformer T41 is connected to a connection point between a first switch Q1 and a second switch Q2, as in the conventional example of FIG. 1.
Moreover, the other end of the resonance capacitor C11 is connected to the other end of the primary winding N41.
In this conventional example of FIG. 3, an AC voltage Vac is converted to an output voltage Vout, as in the conventional example of FIG. 1 and the conventional example of FIG. 2.
A harmonic component of an input current Iin is restrained.
Patent Literature 1: Japanese Patent No. 2,751,961, Specification
Patent Literature 2: Japanese Patent No. 3,367,539, Specification
Patent Literature 3: JP-A-8-182332
Patent Literature 4: JP-A-8-186891
Patent Literature 5: U.S. Pat. No. 5,673,184, Specification
Patent Literature 6: U.S. Pat. No. 5,790,389, Specification
Patent Literature 7: U.S. Pat. No. 6,005,780, Specification
However, in the conventional example of FIG. 1 and the conventional example of FIG. 2, there is a problem that a high power factor cannot be realized with a heavy load.
In the conventional example of FIG. 3, there is a problem that the smoothing capacitor C1 has large voltage stress.